1. Technical Field of the Invention
The present invention relates to an apparatus for transmitting, to a processing unit, data acquired from a plurality of on-vehicle sensor modules (or sensor units) mutually connected by a bus that also connects to the processing unit. Particularly, the present invention relates to an apparatus provided with a plurality of on-vehicle sensor units incorporated in an airbag activation system mounted on a vehicle and configured to detect levels of a collision load in the digital format and a processing unit connected to the sensor modules via a common communication bus that carries the digital data provided by the sensor modules to the processing unit for desired processing thereof.
2. Description of the Related Art
In a conventional airbag activation system, a plurality of sensor modules (sensor units) are used and individually disposed at each part of a vehicle. A signal outputted from each sensor module is converted into digital data of a predetermined format by a communication unit (communication IC) embedded in each sensor module, and is then provided to an electronic control apparatus for airbags (central airbag ECU) by way of buses. This data transmission is called “bus-connected type of sensor data transmission technique,” which is good in simplifying the constructions of a communication system and facilitating system changes. When using this bus-connected type of sensor data transmission technique, transmitting the digital data from each sensor module to the central airbag ECU on a serial signal format is the most suitable for simplifying the communication unit.
As described in the Japanese Patent Laid-open publication No. 06-276570, this bus-connected type of sensor data transmission technique has been well known for an on-vehicle sensor data transmission apparatus as well as a data transmission system dedicated for the airbag activation systems described in the above.
However, a bus transmission band of the bus-connected type of sensor data transmission technique is obliged to be narrow because of, for example, limitations from radiant electromagnetic wave noises. Therefore, the number of sensors that can e connected to one serial bus, the bit length of data that can be given to each sensor module, and a frequency of data output are subjected to strong limitations. For example, the digital data outputted from each sensor module is composed of 4-bit address bits indicating an address of the sensor module, 8-bit data bits following the address bits, and 4-bit error correction bits (or error detection bits for correcting an error) that follow data bits. That is, the digital data is formatted to have 16 bits in total.
In order to reply to a request that radiant electromagnetic wave noises should be lower, the serial bus is usually designed to have a bit rate of 150 kbps or less. Meanwhile, there is a further demand that it is favorable to examine digital data outputted in an airbag activation system at intervals of 500 μs or less, because of keeping an admissible delay for detecting collisions caused between vehicles. This means that the number of sensor modules that can be connected to a single serial bus is limited in the airbag activation system. In addition, it is favorable to keep the bit rate lower in improving noise resistant characteristics and in saving power consumption.
However, in the above airbag activation system and various types of on-vehicle sensor data transmission apparatuses employing the bus-connected type of sensor data transmission method, there has surely been a demand for making it possible that a growing number of sensor modules are connected to the bus. There has also been the demand that the bit length of digital data from each sensor be made greater to ensure higher accuracy thereof.